The Hottest Horizontal - Branch Stars in ω Centauri – Late Hot Flasher vs . Helium Enrichment ⋆

Context. UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as results of the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population recently postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in ω Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C, N enrichment could not be verified. Aims. We want to compare effective temperatures, surface gravities, and abundances of He, C, and N of blue hook and canonical extreme horizontal branch (EHB) star candidates to the predictions of the two scenarios. Methods. Moderately high resolution spectra of stars at the hot end of the blue HB in the globular cluster ω Cen were analysed for atmospheric parameters and abundances using LTE and Non-LTE model atmospheres. Results. In the temperature range 30,000 K to 50,000 K we find that 35% of our stars are helium-poor (log n He n H